DevTools: allow resizing the sub-navigator sidebar in sources panel.
DevTools: allow resizing the sub-navigator sidebar in sources panel.

Change-Id: I2ec5bafdb4ce19336b0cc4279fdab24cfacc4b09
Reviewed-on: https://chromium-review.googlesource.com/1154431
Reviewed-by: Dmitry Gozman <0ca14b32cbb359a30ac562af27dd84cc2813eacb@chromium.org>
Reviewed-by: Aleksey Kozyatinskiy <562acbea96970f508ad9f5cc9fc7c62d82dd3ef8@chromium.org>
Commit-Queue: Pavel Feldman <e1161d7019a0d5c7981e3c3ffc69c76f4b09679a@chromium.org>
Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#579294}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 351b8f7fcc067bb71c25b774a77cd3f9a6132b1d
ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,dart-archive/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,dart-archive/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,dart-archive/devtools-frontend,dart-archive/devtools-frontend,dart-archive/devtools-frontend,dart-archive/devtools-frontend